1. Field of the Disclosure
The present disclosure relates to a crank arm, a crankset, and a power measuring device for an at least partially human powered vehicle or training device with a crank drive.
2. Discussion of the Background Art
By an at least partially human powered vehicle with a crank drive, we understand in particular a common bicycle with a foot pedal crank drive driven by the legs, but also any other at least partially human powered vehicle, in particular a road vehicle, a watercraft or an aircraft, in particular with one or more human powered wheels, which is completely or at least partially driven by the muscles of the user by means of a crank drive, i. e. the propulsion is generated by the rotation of the cranks.
Examples for at least partially human powered vehicles with a crank drive which is not a foot pedal crank drive are bicycles with a hand crank drive, so-called handbikes.
Examples for vehicles which are only partially human powered, are specialized electric bicycles, so-called pedelecs, where an electric motor boosts the pedaling movement of the user in certain riding situations, but cannot completely replace it.
By an at least partially human powered training device with a crank drive, we understand a stationary device for simulating a kind of movement for training purposes which is completely or partially driven by the muscles of the user by means of a crank drive, for example a so-called stationary bicycle for exercising cycling.
The disclosure is described using a bicycle with a common foot pedal crank drive. However, this is not to be understood as a restriction of the usability of the disclosure.
In the field of cycling, both competitive and recreational athletes are interested in recording their current power output. Recording the power output by the cyclist has been established as an inherent part of training supervision and control.
The power generated by the cyclist can be determined from the force exerted on the pedals by the cyclist and the length of the crank arms in connection with the rotational frequency by the physical formula for a rotational movement “power=torque×angular velocity” or “power=torque×2π×rotational frequency”.
For determining the force exerted on the pedals, usually sensors are mounted on or in components of the drive train of the bicycle which are located in the drive train as near to the pedals as possible.
In the prior art, in particular sensors at the pedal itself (WO 2010/014242 A1), at the pedal cleat (WO 2010/000369 A1), at the crank spider or at the crank arm (WO 2008/058164 A2), or at the bottom bracket (DE 37 22 728 C1, DE 299 24 433 U1) are mounted for this purpose. Here, the sensors used are usually strain measurement sensors, in particular strain gages, measuring the deformation of the particular component, from which the force exerted on the component can be derived.
The rotational frequency of the crank arms, i. e. the pedaling frequency, can be measured, for instance, as described in WO 2008/058164 A2, by a Hall sensor fixed to the crank spider or to the crank arm in connection with a magnet fixed to the bicycle frame or indirectly by analyzing the force characteristics measured by the force sensor over time.
In one embodiment (“The Crank Arm Approach”) of WO 2008/058164 A2, it is suggested to place an arrangement of shear web strain gages in the crank arm. To this end, for example pockets are machined into the front and rear side of the crank arm in order to generate a thin web, and the shear web strain gages are arranged on either side thereof. Alternatively, the strain gages are arranged on an outer surface of the crank arm or on an inner surface of a hollow crank arm.
With the arrangement of the sensor in pockets of the crank arm, the problem arises that the crank arm must extensively be mechanically machined and is at the same time structurally weakened. With the arrangement on an outer surface of the crank arm, the sensor is exposed and is therefore unprotected against impacts from outside like weather conditions or mechanical stress like rubbing against the crank arm with the shoes while riding. Furthermore, the sensor is not hidden from the user, which can adversely affect the look of the crank arm and is therefore not desired for design reasons. With the arrangement on an inner surface of a hollow crank arm, the sensor can only be inserted and fixed with difficulties. Sealing problems with the lead-through of the connection lines of the sensor out of the cavity result, and, in this case as well, the sensor is only partially protected against impacts from outside like, for instance, moisture accumulating in the cavity in the crank arm.
The underlying problem of the present disclosure is therefore to provide a crank arm with a sensor, a crankset with such a crank arm, a power measuring system with such a crankset and a bicycle with such a power measuring system, wherein the sensor is protected better against impacts from outside and wherein the crank arm with the sensor can be manufactured easily and cost-efficiently, and to provide a power measuring and displaying method for such a power measuring system.